Neutron detectors are an elemental technology supporting neutron application technologies. With the progress of the neutron application technologies in scientific research fields such as structural analyses by neutron diffraction, nondestructive inspection fields, or security fields such as cargo inspection, there is a demand for a neutron detector which achieves higher performance.
Main performances demanded of the neutron detector are the detection efficiency and the count rate of neutrons, and the ability to discriminate between neutrons and gamma rays (may hereinafter be referred to as n/γ discrimination ability). The detection efficiency means the ratio of the number of radiations counted by the detector to the number of radiations emitted from a radiation source and entered into the detector. The count rate means the number of radiations counted per unit time. Gamma rays are generated when neutrons hit an element contained in a constituent member of a detection system for detecting neutrons, or in an object to be tested, such as Fe (iron), Pb (lead), Cd (cadmium), C (carbon) or N (nitrogen). If the n/γ discrimination ability is low, therefore, a signal which does not reflect the interaction between neutrons and the object to be tested enters, and a so-called background noise increases.
Neutrons have a high permeability to pass through a substance without doing any interaction in the substance. Therefore, a nuclear reaction for promptly converting neutrons into charged particles having energy is generally utilized to detect the neutron beam. For example, a 3He detector which detects neutrons by utilization of protons or tritons generated by a nuclear reaction between helium-3 (3He) and neutrons has so far been known. This detector has high detection efficiency and excellent n/γ discrimination ability, but has posed the problem of a limited count rate. Moreover, helium-3 is an expensive substance and its resources are limited.
Recently, the development of a detector using a neutron scintillator, instead of the above-mentioned 3He detector, has been underway in an attempt to obtain an inexpensive and upsized detector. The neutron scintillator refers to a substance which, when hit by neutrons, absorbs the neutrons to emit fluorescence. The aforementioned various performances of a neutron detector using this scintillator depend on a substance constituting the scintillator. For example, the magnitude of the light emission intensity of the scintillator or variations in its light emission intensity affect discrimination from a background noise. Since the background noise is mainly ascribed to gamma rays, the n/γ discrimination ability is influenced. The decay time of fluorescence influences the count rate.
LiF/ZnS has been used as a neutron scintillator having a relatively high neutron detection efficiency and excellent n/γ discrimination ability (see Non-Patent Document 1). Since the LiF/ZnS is opaque, however, an increase in its thickness has made it impossible to take out scintillation light (scintillation) efficiently. Thus, the LiF/ZnS has been limited in the improvement of neutron detection efficiency.
In view of such problems, a proposal has been made for a neutron scintillator using a eutectic body composed of europium-containing calcium fluoride crystals and lithium fluoride crystals (see Non-Patent Document 2). This neutron scintillator is translucent, and enables scintillation light to be collected efficiently. Thus, this neutron scintillator can achieve a very high neutron detection efficiency. According to studies by the inventors of the present invention, however, the neutron scintillator at issue involves great variations in the brightness of scintillation light when detecting neutrons, and thus has posed difficulty in discriminating between neutrons and gamma rays.
As a eutectic body similar to the eutectic body constituting the neutron scintillator of the present invention, a disclosure is made of a eutectic body having manganese incorporated into a eutectic body composed of lithium fluoride crystals and calcium fluoride crystals (see Non-Patent Document 3). The eutectic body is described as being intended for the development of a dosimeter utilizing thermoluminescence phenomena, and the thickness of laminar lithium fluoride crystal layers is described as about 5 μm. However, the details of the layered structure of the eutectic body remain unknown. Nor has any study been conducted on the application of the eutectic body as a neutron scintillator, and no findings have been presented in connection with the thickness of the crystal layers and the layered structure preferred for a neutron scintillator.